Synthesis and characterization of Au–Fe alloy nanoparticles embedded in a silica matrix by atom beam sputtering

Abstract

In the present study, we investigated the formation of AuFe alloy nanoparticles embedded in a silica matrix by the cosputtering of silica, Au and Fe with two different metal fractions using an atom beam source. The increase in the metal fraction in the thin film results in the formation of AuFe alloy nanoparticles. The absence of surface plasmon resonance peak for the Au nanoparticles in the optical spectra, structural studies and transmission electron microscopy results confirmed the existence of AuFe alloy nanoparticles. The nanocomposite is ferromagnetic at 2 K with a symmetric hysteresis loop. The formation of AuFe alloy nanoparticles in the thin film is explained on the basis of interatomic distance and diffusion during deposition.

---

1. Introduction

Bimetallic nanoparticles have different chemical and physical properties and superior features than their monometallic counterparts. In recent research scenario, bimetallic nanoparticles (NPs) have attracted substantial attention due to their manifold potential for application in several fields such as catalysis, biosensing, energy storage, data storage, magnetic resonance imaging, and aerospace technology.^1–6 The enhanced catalytic properties of bimetallic nanoparticles have aroused significant interest in the field of chemical catalysis.⁷ Bimetallic nanoparticles may form different structures, such as core shell structures, heterostructures and alloy nanoparticles, depending on the metal, relative concentration, and synthesis method used.^8,9 It has been reported by several groups that a solid–solid interface at the surface of an embedded nanoparticle provides extra degrees of control over the properties of the nanoparticles.^10–12 For example, contrary to the lower melting point observed for free standing clusters, Ge nanocrystals embedded in a silica matrix show increase in melting point of nearly 200 K above the bulk value.¹³ Metallic nanoparticles embedded in glass can increase the third order optical susceptibility of the composites by several orders of magnitude, making such materials an interesting candidate for optical switches.^14,15

Most of the studies on bimetallic nanoparticles dispersed in a matrix involve metallic elements that are miscible. There are very few reports on metallic elements that are immiscible in the bulk phase. In the case of Au–Pt bulk phase diagram, a miscible gap exists for Au–Pt bulk alloy.¹⁶ In spite of this, the Au–Pt alloy nanoparticles can be synthesized in the whole composition range, which demonstrates that the alloying mechanism¹⁷ of nanosized materials is quite different from those of their bulk counterparts. Recently, it has also been shown that the cosputtering of Co and Cu metals, which are immiscible in the bulk phase, in a silica matrix forms alloy nanoparticles.¹⁸ Therefore, in the present study, our interest is to investigate the bimetallic system of Au and Fe; these metals are immiscible in the bulk phase. The pure Fe nanoparticles are magnetic in character and prone to oxidation, which may lead to the deterioration of their magnetic properties. Au nanoparticles offer plasmonic properties that are tunable depending on their size, shape and surrounding medium.¹⁹ Au–Fe alloy nanoparticles could possess both optical and magnetic properties. Combining the optical and magnetic properties of Au and Fe in a single nanostructure would be useful in various applications such as nanomedicine,^20,21 information technology^22–24 and catalysis.²⁵

At the nanoscale, attempts to synthesize AuFe alloy have been limited so far. The formation of AuFe alloy nanoparticles was reported by Mattei et al. using an ion implantation technique in which Au and Fe were sequentially implanted in silica matrix.²⁶ AuFe nanoparticles obtained by sequential ion implantation were found to have problems with surface accessibility,^27,28 which is important for catalytic applications.²⁴ In catalytic applications, surface accessibility (the surface area accessible to the reactants) is directly related to the rate of product formation in a reaction; it follows that the greater the amount of surface area accessible to the reactants, the larger the throughput will be.²⁹ Therefore, a new approach is needed that allows the synthesis of well dispersed and accessible nanoparticles. In the present study, we describe the synthesis of AuFe alloy nanoparticles dispersed in a silica matrix using an atom beam sputtering technique. X-ray photoelectron spectroscopy (XPS), X-ray diffraction, UV-visible spectroscopy and transmission electron microscopy have been carried out to confirm the existence of Au–Fe alloy nanoparticles in the silica matrix.

2. Experimental details

In the present study, Au and Fe were cosputtered along with silica for the synthesis of nanocomposite thin films of silica containing metallic nanoparticles using an atom beam sputtering technique.^30–32 A schematic of the atom beam sputtering setup with geometrical details is shown in Fig. 1. It consists of a 1 keV argon atom source, which is placed at an angle of 45° facing in the direction of the sputtering target. The distance between the Ar atom source and the sputtering target is 150 mm. A 3 inch diameter disc of pure SiO₂ with foils of Au and Fe glued on it was used as the sputtering target. The details of the source, purity and thickness of the Au and Fe foils used are given in Table 1. The substrate holder was placed at an angle of 45° facing toward the target surface; the distance between the target and the substrate holder was 150 mm. The substrate holder as well as the target holder was rotated with the help of a DC motor at a speed of 5 rotations per minute for uniform sputtering and deposition of the thin films. The base pressure and the sputter pressure of the chamber were 7 × 10⁻⁷ mbar and 4.6 × 10⁻³ mbar, respectively. The atom source supplies a current of 30 mA on the target. The relative area of the silica disc and the Au and Fe foils exposed to the atom beam determines the size of the metal fractions in the thin film. Initially, a set of nanocomposite thin films with a 16 at% metal fraction was deposited on a quartz substrate. Subsequently, a second set of nanocomposite thin films on a silicon substrate with a 33 at% metal fraction was prepared by increasing the relative area covered by the Au and Fe foils on the silica disc. The above mentioned metal fraction represents the total composition of the percentages of Au and Fe in the thin films. For convenience, hereafter the nanocomposite thin films with 16 at% and 33 at% metal fractions are designated as “A” and “B”, respectively.

Fig. 1 Schematic of the atom beam sputtering setup.

Table 1 Details of the source, purity and thickness of the Au and Fe foils used

Foil used	Area of foil (mm^2)	Total number of foils used	Thickness (mm)	Purity (%)	Source
Au	25	16	0.25	99.95	Aldrich
Fe	25	16	0.1	99.99	Aldrich

---

The thickness and metal content of the thin films were determined by Rutherford backscattering spectrometry (RBS) using a 1.7 MeV tandem accelerator facility with 2 MeV He⁺ ions. A silicon surface barrier detector was used at a backscattering angle of 165° and a solid angle of 1 msr. High resolution X-ray diffraction (XRD) measurements of the thin films were carried out at Elletra (Italy) using a synchrotron source with X-ray energy of 15 keV to identify the crystalline phases. The measurements were performed in θ–2θ geometry with a step size of 0.005° sec⁻¹. X-rays from the synchrotron source were used, as the conventional XRD (Cu Kα source) did not provide intense, sharp and well resolved data due to the low intensity of the X-rays and limited resolution of the setup. Optical absorbance spectroscopy of the thin films was carried out in the wavelength range of 200 –800 nm. The optical spectra were obtained in absorbance mode for the thin film A deposited on a quartz substrate and in reflectance mode for the thin film B deposited on a Si substrate, which are not transparent. The reflectance spectra were converted into absorbance spectra using apparent absorbance log(1/R).³³ This provides a good representation of absorbance spectra. X-ray photoelectron spectroscopy (XPS) was carried out using a PHI 5000 Verse Probe II system. Microfocused Al Kα radiation (1486.6 eV) at 100 W power was used for this study. Under these conditions, the full width at half maximum of the Ag 3d_5/2 line from a standard Ag sample was about 0.6 eV. The binding energy scale was referenced to the C 1s peak at 284.5 eV to correct the charging effects. For fitting the XPS spectra, the Shirley method of background correction was used. Transmission electron microscopy (TEM) study of the thin films was carried out with 200 keV electrons using the JEOL JEM – 2010 (UHR) facility. The magnetic properties were measured at 2 K using a vibrating sample magnetometer (VSM) and applying a maximum field of 2 Tesla.

3. Results

It is well known that the properties of nanocomposite thin films are highly correlated with the composition of the films. In the present study, RBS as well as SRIM code³⁴ (Stopping and Range of Ions in Matter) were employed to determine the composition of the nanocomposite thin films. The RBS spectra and depth profiles of the nanocomposite thin films A and B are shown in Fig. 2 and 3, respectively. The estimation of the metal fractions and depth distribution was performed by simulation of the RBS spectra using Rutherford Universal Manipulation Program (RUMP) simulation code.³⁵ The total thicknesses of the nanocomposite thin films and the Au and Fe atomic metal fractions are shown in Table 2. The thicknesses of nanocomposite thin films A and B were 520 nm and 450 nm, respectively.

Fig. 2 RBS spectra of (a) thin film A, (b) depth profile of different elements in thin film A.

Fig. 3 RBS spectra of (a) thin film B, (b) depth profile of different elements in thin film B.

Table 2 Thickness and atomic fraction of thin films calculated using RUMP simulation of the RBS spectra

Thin film	Thickness (nm)	Atomic fraction (%)
A	520	Au	7.4
		Fe	8.8
		SiO2	83.8
B	450	Au	19.2
		Fe	14.0
		SiO2	66.8

---

To theoretically calculate the composition of the films, the sputtering yield of Au, Fe and SiO₂ was estimated from SRIM code. SRIM code is based on the Monte Carlo simulation method. The required input sputtering parameters are the incident ion (Ar), incident ion energy (1 keV), incident angle (45°) and total number of ions (10 000). The sputtering yields (Y) of Au, Fe, Si and O calculated using SRIM code are shown in Table 2. The atomic fraction of an element (Au and Fe) in a nanocomposite film is calculated using the equation³⁶

where i stands for the element (Au, Fe, Si and O) and m stands for metals only. The atomic fractions of Au and Fe calculated using eqn (1) in comparison with the RBS results are shown in Table 3. It is clear from the SRIM and RBS results that the measurements performed using SRIM code overestimate the sputtering yields of the elements.

Table 3 Sputtering yields and atomic fractions of different elements using SRIM code simulation

Thin film	Element	Area Ai (mm^2)	Sputtering yield Yi (atoms/ion)	Atomic fraction (%)
A	Au	150	4.3	8.6
	Fe	200	3.5	9.4
	SiO2	1678	0.8 (Si), 2.9 (O)	82.0
B	Au	400	4.3	22.7
	Fe	400	3.5	18.5
	SiO2	1228	0.8 (Si), 2.9 (O)	58.8

---

The UV-visible absorption spectra of the nanocomposite thin films A and B are shown in Fig. 4(a) and (b), respectively. Both spectra were obtained with a bare substrate as a reference. The spectrum in Fig. 4(a) contains a faint surface plasmon resonance (SPR) band at a wavelength of about 525 nm, and the spectrum in Fig. 4(b) does not show any SPR band. Basically, SPR bands are derived from the collective excitation of conduction electrons in nanoscale noble metals by annihilation of the incident photons; their resonance energy depends on the size, shape and interparticle separation of the nanoparticles, as well as on the chemical and physical environment.²³ It is well known that pure Au nanoparticles in silica exhibit an SPR band at about 530 nm,³⁷ whereas Fe nanoparticles do not have a SPR in the visible range. Mattei et al.²⁶ reported the sequential ion implantation of Au and Fe in silica and observed that due to AuFe alloying, the optical absorption spectrum does not exhibit the SPR of Au nanoparticles. In thin film A, the presence of a faint Au SPR band indicates the presence of isolated Au nanoparticles without any interaction with Fe atoms, thus indicating no alloy formation. The absence of SPR peak for Au nanoparticles in thin film B indicates that a strong electronic interaction between Au and Fe atoms occurred, which enabled damping of the Au SPR band. Keeping in mind the interaction between Au and Fe atoms in thin film B, AuFe alloy formation is inferred, whereas in the case of thin film A, there was no alloy formation during deposition.

Fig. 4 UV-visible absorption spectra of (a) thin film A and (b) thin film B.

Furthermore, to confirm the alloy formation, XPS measurements were obtained using Al Kα radiation. The survey spectra of thin film A and thin film B displayed in Fig. 5 reveal the presence of Au and Fe in both the films. The peak at 284.5 eV is the C 1s peak, which is attributed to the unavoidable presence of hydrocarbons on the sample surface; all the XPS spectra, as mentioned earlier, have been charge referenced to this peak. The high resolution XPS spectra for thin film A and thin film B in the Au 4f and Fe 2p regions are shown in Fig. 6 and 7, respectively. Deconvoluting the Au (4f) region in Fig. 6(a) shows that the main peak is located at 84.05 eV, the value of elemental Au. The other small peak is positioned at 85 eV, attributed to small Au clusters with diameters smaller than 2 nm.³⁸ For large clusters of diameters higher than 3 nm, the binding energy closely approximated the bulk metal value.³⁹ There is no signature of AuFe alloy in the spectrum. It is expected that due to the low metal fraction in the silica matrix, Au and Fe interaction does not take place during deposition. For the case of thin film B, deconvoluting the Au (4f) peak by curve fitting shows the presence of pure Au as well as an additional peak having 0.6 eV lower binding energy (as shown in Fig. 6(b)). This is attributed to negative charge buildup on the Au atoms upon AuFe alloy formation as a result of Au (2.54) being more electronegative than Fe (1.83). Luo et al.⁴⁰ reported the formation of AuPd alloy and observed a negative shift of 0.2 eV in the binding energy of the Au 4f_7/2 peak on alloying. Therefore, in the present case, an additional peak in the Au 4f region in Fig. 6(b) was ascribed to AuFe alloy. The intensity of the AuFe alloy peak indicates that the alloyed fraction of Au with Fe is more than unalloyed pure Au. XPS peak fit parameters for thin film A as well as thin film B are given in Table 4. The XPS spectra in the Fe 2p region in Fig. 7 show the presence of Fe₂O₃ in thin film B.

Fig. 5 A XPS survey spectra for thin film A and thin film B.

Fig. 6 XPS spectra in the Au 4f region of (a) thin film B and (b) thin film A.

Fig. 7 XPS spectrum in the Fe 2p region of (a) thin film A and (b) thin film B.

Table 4 XPS peak fit parameters of thin film A and thin film B

Thin film		Peak 1 (eV)		Peak 2 (eV)
		Centre	FWHM	Centre	FWHM
A	4f7/2	84	1.2	85	1.18
	4f5/2	87.7	1.2	88.75	1.2
B	4f7/2	83.48	1.1	84	1.13
	4f5/2	87.17	1.1	87.70	1.13

---

For morphological studies of the AuFe alloy in thin film B, TEM measurements were carried out on the thin film. The morphology of the nanoparticles is shown in Fig. 8(a), and the corresponding nanoparticle size distribution is shown in Fig. 8(b). The average nanoparticle size is around 3 nm. All nanoparticles in the thin films are more or less spherical in shape. Although some of the nanoparticles appear to be overlaying and interconnected, this is not really the situation; in planar TEM imaging, projections of all nanoparticles from different depths appear in the image. To determine the composition of these nanoparticles, EDX (energy dispersive X-ray) elemental mapping was performed on nanoparticles by selecting the Au peak edge (83 eV) and the Fe peak edge (708 eV). The Au and Fe elemental mapping image is shown in Fig. 8(c). The results show that Fe atoms are present in the silica matrix as well as in the nanoparticles, along with Au atoms. The Fe atoms present in the silica matrix may have formed iron oxide.

Fig. 8 TEM of thin film B (a) image, (b) nanoparticle size distribution and (c) elemental mapping using EDS.

For structural studies of thin film B, XRD was performed using synchrotron radiations. The diffraction pattern of the nanocomposite thin film B is shown in Fig. 9. The diffraction pattern shows a face centered cubic (fcc) structure analogous to pure Au (111) at a 2θ angle of 20.15° and a peak corresponding to Fe₂O₃. The other reflection in the thin film spectra is at a slightly lower angle than the pure Au (111) peak position. The lattice parameter of this peak is 0.412 nm, which is slightly higher than the lattice parameter of pure Au (0.407 nm). This lattice parameter is also higher than that expected by the simple rule of mixing Au and Fe (Vegard's law). The higher lattice parameter of AuFe alloy nanoparticles can be explained in terms of self-interstitials.⁴¹ A self-interstitial is a type of point defect wherein a lattice atom occupies an interstitial site instead of its regular position.⁴² Any interstitial site is smaller than its own atom size, and the presence of an atom at such an interstitial site results in strain in the lattice surrounding it.

Fig. 9 XRD pattern of nanocomposite thin film B.

It is clear from the XRD pattern in Fig. 9 that the as-deposited AuFe alloy is Au rich. Some of the Fe atoms are present in the Au FCC lattice on the substitutional sites, which in principle contracts the lattice parameters due to the smaller size of the Fe atoms. However, in the present case, the observed high lattice parameter also suggests the presence of self-interstitials. Most of the Fe in the thin film, which does not take part in the alloy, is present in the oxide form (Fe₂O₃), as shown in Fig. 9, which matches well with the observations of the XPS spectra in Fig. 7 and the TEM image in Fig. 8.

The magnetic field dependence of the magnetization (M) of AuFe alloy nanoparticles in thin film B was measured using VSM at a temperature of 2 K. Fig. 10 shows the variation of magnetization M as a function of external magnetic field H. The spectra were corrected for the diamagnetic contribution of the silicon substrate and normalized to the total number of Fe atoms in thin film B measured using RBS. The magnetization versus magnetic field plot measured at a temperature of 2 K (Fig. 10) shows that M linearly increases with applied magnetic field with a symmetric hysteresis loop, showing the ferromagnetic behavior of AuFe alloy. The inset of Fig. 10 shows the enlarged view of the hysteresis loop from the centre. The coercivity and retentivity of the hysteresis loop are 53 Oe and 5.6 × 10⁻⁴ Am² g⁻¹, respectively. Guire et al.⁴³ measured the dependence of the magnetic moment per Fe atom as a function of Au_xFe_100−x composition in melt spinning alloys and observed a decrease in magnetic moment per Fe atom with increasing Au content in the alloy. Amendola et al.⁴⁴ reported the value of the magnetic moment per Fe atom as 0.01 Am² g⁻¹ for Au₈₉Fe₁₁ nanoalloys, which is in close agreement with the present results and suggests that the composition of the AuFe alloy obtained in the present study should be enriched in Au.

Fig. 10 M − H curve of the nanocomposite thin film B measured at 2 K.

4. Discussion

To understand the formation and growth of nanoparticles in thin films A and B, we began by evaluating the flux of sputtered species (Au, Fe, SiO₂) arriving at the substrate. It has been reported that the angular distribution of the ejected species generally follows a nearly cosine distribution, although over/under cosine distribution may be observed depending on the energy of incident Ar atoms, the angle of incidence and the target type.^45–47 In the present case, the Ar atom beam source bombarded the target with a flux of 5 × 10¹⁵ atoms cm⁻² s⁻¹, corresponding to a beam current of 30 mA. The incident flux of the sputtered species reaching to the substrate surface is an important parameter, as it decides the effective time the surface atoms have to diffuse before they are covered by the subsequent arriving species. The incident flux of the sputtered species can be estimated by sputtering yield; however, a fraction of the sputtered species is desorbed after reaching the substrate surface, and therefore it is difficult to calculate the actual incident flux. Apart from this, the scattered Ar atom of energy 1 keV may also hit the substrate/film. However, the incident flux can be estimated quite well from the film deposition rate. In the present case, the film deposition rate is a few tens of monolayers per minute, and thus the incident flux of metallic species on the substrate surface is about 6 × 10¹³ atoms cm⁻² s⁻¹. The kinetic energy of these sputtered species influences the structure and morphology of the thin film. Kinematic theory shows that higher the mass of the sputtered species, the lower its kinetic energy will be. The mean kinetic energy of Au and Fe sputtered atoms calculated by TRIM code is 20 eV. This energy is dissipated in the thin film and increases the local transient temperature of the film at the landing points. This temperature rise may lead to a transiently spatiotemporal enhancement of surface migration of adatoms and chemical interaction between the landing species. Khan et al.⁴⁸ indicated that the species of incident energy of 1–100 eV can create a transient temperature rise of 1000 K in a radius of 0.5–2 nm for a few picoseconds.

The growth of a thin film using atom beam sputtering takes into account the following basic processes: (i) absorption and desorption of atoms on the substrate surface, (ii) diffusion of deposited atoms on the surface, and (iii) clustering of atoms leading to the nucleation and growth of nanoparticles.⁴⁹ However, the formation of nanoparticles depends on the average interatomic distance (d_avg) of the atoms in the thin film, which is a function of the areal coverage of the Au and Fe foils on the SiO₂ target. The interatomic distance is calculated using the RBS results. In the present calculation, it is assumed that the Au and Fe atoms in the thin film are identical and point like objects. If we take an atom as the centre and draw a sphere of radius r such that it is in contact with a maximum number of like spheres, the available space is filled very efficiently. In this way, the interatomic distance is d. The value of d can be evaluated as

N × volume of a sphere = volume of thin film (2)

where N is the total number of atoms in the thin film. Using eqn (1), the estimated values of d are 0.48 nm and 0.63 nm for thin films A and B, respectively. The nucleation and growth of nanoparticles occurs when

d < L_d

where L_d is the diffusion length of the metal atoms. The relative diffusion length (L_d) of the adatoms can be calculated as⁵⁰where D is the diffusion coefficient, θ_m is the flux of incident metallic species on the substrate surface, E is the activation energy of the metal atom on the SiO₂ surface,⁴⁸ R is the gas constant and T is the substrate temperature. In the present case, the calculated diffusion length L_d of the Au atoms on the surface of SiO₂ is ≈0.5 nm. For simplicity, it is assumed that the diffusion length of Fe atoms on the surface of SiO₂ is also ≈0.5 nm. Au, Fe and SiO₂ are all present in thin films A and B; therefore, the following possibilities of interaction arise: (a) Au, Au (b) Au, Fe (c) Fe, Fe (d) Si, O and (e) Fe, O.

The bond energies between Au–Au, Au–Fe, Fe–Fe, Si–O, and Fe–O atoms are 222, 187, 100, 809 and 408 kJ mol⁻¹, respectively.⁵¹ During sputtering, sputtered species may be in an atomic or cluster form. Atoms landing on the surface are quite mobile and diffuse over the surface. These diffusing adatoms prefer sites with higher binding energies, because this helps to lower the potential energy of the system. Most of the Si and O atoms reaching the surface form SiO₂ molecules by surface reaction due to the highest bond energy. Au atoms diffusing on the surface of the silica rich region will prefer to attach to Au adatoms due to the higher Au–Au bond energy compared to the Au–Fe bond energy. If such sites are unavailable in the close vicinity, then the Au adatom is trapped at the nearest Fe site; therefore, the AuFe alloy cluster nucleates. The further growth of Au clusters and alloy clusters takes place due to subsequent Au and Fe adatoms reaching close to these clusters before being buried under subsequent silica deposits, because the silica concentration is higher. It may be mentioned here that Au adatoms can also interact with Si atoms; however, the availability of unreacted Si atoms on the surface is very low due to the strong tendency of formation of SiO₂ molecules. Similarly, diffusing Fe adatoms may react with O, Au and Fe with decreasing probability in that order. Fe adatoms have a stronger tendency to bond with nearby oxygen atoms to form iron oxide.

In the case of set A, due to the higher interatomic distance as compared to the diffusion length, the probability of interaction of Au and Fe atoms with each other is much smaller. Even if some of these atoms come in close proximity, Au–Au bonds form preferentially compared to Au–Fe bonds due to the higher bond energy. This suggests that small Au clusters nucleate during deposition (as confirmed by the faint Au SPR peak in Fig. 4(a) and the XPS spectrum in Fig. 6(a)), and AuFe alloy nanoparticles are not formed or if they are formed, they are too small and few in number to be detected by various characterization techniques.

In the case of set B, due to the smaller interatomic distance as compared to set A, the probability of the interaction of atoms is high. This suggests the formation of AuFe alloy nanoparticles along with Au nanoparticles during deposition, which is quite evident from the XPS spectrum in Fig. 6(b) and the XRD pattern in Fig. 9.

5. Conclusion

The synthesis of thin films containing AuFe alloy nanoparticles embedded in a silica matrix by an atom beam sputtering technique has been studied. Two different metal fractions were cosputtered with silica. The composition and average interatomic distances between metal atoms in the thin films were determined by RBS. It has been found that thin films with interatomic distances less than the diffusion length of the metal atoms result in the formation of AuFe alloy nanoparticles. Optical spectroscopy, XPS and XRD results confirmed the existence of the AuFe alloy nanoparticles in the thin film. The AuFe alloy in the thin film is observed to be magnetic in nature. The Au and Fe interatomic distance and diffusion during deposition play a crucial role in the formation of AuFe alloy nanoparticles.

Acknowledgements

One of the authors (Compesh Pannu) would like to acknowledge University Grant Commission, India for providing financial support in the form of a fellowship. The author also thanks Dr Santanu Ghosh, IIT Delhi for reflectance spectroscopy and AIRF, JNU for providing TEM facility and VSM measurements.

References

1. R. Ferrando, J. Jellinek and R. L. Johnston, Nanoalloys: from theory to applications of alloy clusters and nanoparticles, Chem. Rev., 2008, 108, 845–910.
2. S. Koh, J. Leisch, M. F. Toney and P. Strasser, Structure-activity-stability relationships of Pt–Co alloy electrocatalysts in gas-diffusion electrode layers, J. Phys. Chem. C, 2007, 111, 3744–3752.
3. B. Lim, M. Jiang, P. H. Camargo, E. C. Cho, J. Tao, X. Lu, Y. Zhu and Y. Xia, Pd–Pt bimetallic nanodendrites with high activity for oxygen reduction, Science, 2009, 324, 1302–1305.
4. G. Gupta, D. A. Slanac, P. Kumar, J. D. Wiggins-Camacho, X. Wang, S. Swinnea, K. L. More, S. Dai, K. J. Stevenson and K. P. Johnston, Highly stable and active Pt− Cu oxygen reduction electrocatalysts based on mesoporous graphitic carbon supports, Chem. Mater., 2009, 21, 4515–4526.
5. J. Greeley, I. Stephens, A. Bondarenko, T. P. Johansson, H. A. Hansen, T. Jaramillo, J. Rossmeisl, I. Chorkendorff and J. K. Nørskov, Alloys of platinum and early transition metals as oxygen reduction electrocatalysts, Nat. Chem., 2009, 1, 552–556.
6. S. Khanal, G. Casillas, J. J. Velazquez-Salazar, A. Ponce and M. Jose-Yacaman, Atomic Resolution Imaging of Polyhedral PtPd Core–Shell Nanoparticles by Cs-Corrected STEM, J. Phys. Chem. C, 2012, 116, 23596–23602.
7. N. Bhattarai, S. Khanal, D. Bahena, R. L. Whetten and M. Jose-Yacaman, in Synthesis and Structural Characterization of Ferromagnetic Au/Co Nanoparticles, MRS Proceedings, Cambridge Univ Press, 2014, pp mrss14-1708-vv06-03.
8. M. Einax, W. Dieterich and P. Maass, Colloquium: cluster growth on surfaces: densities, size distributions, and morphologies, Rev. Mod. Phys., 2013, 85, 921.
9. S. Khanal, A. Spitale, N. Bhattarai, D. Bahena, J. J. Velazquez-Salazar, S. Mejía-Rosales, M. M. Mariscal and M. José-Yacaman, Synthesis, characterization, and growth simulations of Cu–Pt bimetallic nanoclusters, Beilstein J. Nanotechnol., 2014, 5, 1371–1379.
10. E. Sutter and P. Sutter, Phase diagram of nanoscale alloy particles used for vapor–liquid–solid growth of semiconductor nanowires, Nano Lett., 2008, 8, 411–414.
11. V. C. Holmberg, M. G. Panthani and B. A. Korgel, Phase transitions, melting dynamics, and solid-state diffusion in a nano test tube, Science, 2009, 326, 405–407.
12. B. Kim, J. Tersoff, S. Kodambaka, M. Reuter, E. Stach and F. Ross, Kinetics of individual nucleation events observed in nanoscale vapor–liquid–solid growth, Science, 2008, 322, 1070–1073.
13. Q. Xu, I. Sharp, C. Yuan, D. Yi, C. Liao, A. Glaeser, A. Minor, J. Beeman, M. Ridgway and P. Kluth, Large melting-point hysteresis of Ge nanocrystals embedded in SiO 2, Phys. Rev. Lett., 2006, 97, 155701.
14. P. Mazzoldi, G. Arnold, G. Battaglin, F. Gonella and R. Haglund Jr, Metal nanocluster formation by ion implantation in silicate glasses: nonlinear optical applications, J. Nonlinear Opt. Phys. Mater., 1996, 5, 285–330.
15. U. Kreibig and M. Vollmer, Optical properties of metal clusters, 1995.
16. R. Hultgren, P. D. Desai, D. T. Hawkins, M. Gleiser and K. K. Kelley, Selected values of the thermodynamic properties of binary alloys; DTIC Document, 1973.
17. S. Xiao, W. Hu, W. Luo, Y. Wu, X. Li and H. Deng, Size effect on alloying ability and phase stability of immiscible bimetallic nanoparticles, Eur. Phys. J. B, 2006, 54, 479–484.
18. G. Mattei, G. Battaglin, E. Cattaruzza, C. Maurizio, P. Mazzoldi, C. Sada and B. Scremin, Synthesis by co-sputtering of Au–Cu alloy nanoclusters in silica, J. Non-Cryst. Solids, 2007, 353, 697–702.
19. K. L. Kelly, E. Coronado, L. L. Zhao and G. C. Schatz, The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment, J. Phys. Chem. B, 2003, 107, 668–677.
20. J. R. McCarthy and R. Weissleder, Multifunctional magnetic nanoparticles for targeted imaging and therapy, Adv. Drug Delivery Rev., 2008, 60, 1241–1251.
21. J. Gao, H. Gu and B. Xu, Multifunctional magnetic nanoparticles: design, synthesis, and biomedical applications, Acc. Chem. Res., 2009, 42, 1097–1107.
22. V. Belotelov, I. Akimov, M. Pohl, V. Kotov, S. Kasture, A. Vengurlekar, A. V. Gopal, D. Yakovlev, A. Zvezdin and M. Bayer, Enhanced magneto-optical effects in magnetoplasmonic crystals, Nat. Nanotechnol., 2011, 6, 370–376.
23. P. K. Jain, Y. Xiao, R. Walsworth and A. E. Cohen, Surface plasmon resonance enhanced magneto-optics (SuPREMO): Faraday rotation enhancement in gold-coated iron oxide nanocrystals, Nano Lett., 2009, 9, 1644–1650.
24. L. Wang, C. Clavero, Z. Huba, K. J. Carroll, E. E. Carpenter, D. Gu and R. A. Lukaszew, Plasmonics and Enhanced Magneto-Optics in Core−Shell Co−Ag Nanoparticles, Nano Lett., 2011, 11, 1237–1240.
25. D. Wang and Y. Li, Bimetallic Nanocrystals: Liquid-Phase Synthesis and Catalytic Applications, Adv. Mater., 2011, 23, 1044–1060.
26. G. Mattei, C. de Julián Fernández, G. Battaglin, C. Maurizio, P. Mazzoldi and C. Scian, Structure and thermal stability of Au–Fe alloy nanoclusters formed by sequential ion implantation in silica, Nucl. Instrum. Methods Phys. Res., Sect. B, 2006, 250, 225–228.
27. C. de Julián Fernández, G. Mattei, E. Paz, R. Novak, L. Cavigli, L. Bogani, F. Palomares, P. Mazzoldi and A. Caneschi, Coupling between magnetic and optical properties of stable Au–Fe solid solution nanoparticles, Nanotechnology, 2010, 21, 165701.
28. I. C. Chiang and D. H. Chen, Synthesis of monodisperse FeAu nanoparticles with tunable magnetic and optical properties, Adv. Funct. Mater., 2007, 17, 1311–1316.
29. C. C. Koch, Nanostructured materials: processing, properties and applications, William Andrew, 2006.
30. D. Avasthi, Y. Mishra, D. Kabiraj, N. Lalla and J. Pivin, Synthesis of metal–polymer nanocomposite for optical applications, Nanotechnology, 2007, 18, 125604.
31. D. Kabiraj, S. Abhilash, L. Vanmarcke, N. Cinausero, J. Pivin and D. Avasthi, Atom beam sputtering setup for growth of metal particles in silica, Nucl. Instrum. Methods Phys. Res., Sect. B, 2006, 244, 100–104.
32. C. Pannu, U. B. Singh, D. C. Agarwal, S. A. Khan, S. Ojha, R. Chandra, H. Amekura, D. Kabiraj and D. K. Avasthi, A study on the consequence of swift heavy ion irradiation of Zn–silica nanocomposite thin films: electronic sputtering, Beilstein J. Nanotechnol., 2014, 5, 1691–1698.
33. S. Kelly, D. Hesterberg and B. Ravel, Methods of soil analysis, Part 5, Mineralogical methods, Soil Science Society of America, Medison, 2008, pp. 387–463.
34. J. F. Ziegler and J. P. Biersack, SRIM-2008, Stopping Power and Range of Ions in Matter, 2008.
35. L. R. Doolittle, A semiautomatic algorithm for Rutherford backscattering analysis, Nucl. Instrum. Methods Phys. Res., Sect. B, 1986, 15, 227–231.
36. H. Kumar, Y. Mishra, S. Mohapatra, D. Kabiraj, J. Pivin, S. Ghosh and D. Avasthi, Compositional analysis of atom beam co-sputtered metal–silica nanocomposites by Rutherford backscattering spectrometry, Nucl. Instrum. Methods Phys. Res., Sect. B, 2008, 266, 1511–1516.
37. C. Kan, W. Cai, C. Li, L. Zhang and H. Hofmeister, Ultrasonic synthesis and optical properties of Au/Pd bimetallic nanoparticles in ethylene glycol, J. Phys. D: Appl. Phys., 2003, 36, 1609.
38. C. Xu, T. Sritharan, S. Mhaisalkar, M. Srinivasan and S. Zhang, An XPS study of Al 2 Au and AlAu 4 intermetallic oxidation, Appl. Surf. Sci., 2007, 253, 6217–6221.
39. D. C. Lim, I. Lopez-Salido, R. Dietsche, M. Bubek and Y. D. Kim, Size-Selectivity in the Oxidation Behaviors of Au Nanoparticles, Angew. Chem., Int. Ed., 2006, 45, 2413–2415.
40. W. Luo, M. Sankar, A. M. Beale, Q. He, C. J. Kiely, P. C. Bruijnincx and B. M. Weckhuysen, High performing and stable supported nano-alloys for the catalytic hydrogenation of levulinic acid to γ-valerolactone, Nat. Commun., 2015, 6, 6540.
41. P. Mukherjee, Crystal structures and phase formation thermodynamics of iron–gold nanoclusters, 2013.
42. K. L. Murty and I. Charit, An introduction to nuclear materials: fundamentals and applications, John Wiley & Sons, 2013.
43. T. McGuire, J. Aboaf and E. Klokholm, Magnetic and transport properties of Fe–Au and Co–Au films, J. Appl. Phys., 1981, 52, 2205–2207.
44. V. Amendola, M. Meneghetti, O. M. Bakr, P. Riello, S. Polizzi, D. H. Anjum, S. Fiameni, P. Arosio, T. Orlando and C. de Julian Fernandez, Coexistence of plasmonic and magnetic properties in Au₈₉Fe ₁₁ nanoalloys, Nanoscale, 2013, 5, 5611–5619.
45. B. Emmoth and M. Braun, Angular Distributions of Sputtered Si and SiO₂ Combined with Surface Morphology Studies, Phys. Scr., 1981, 24, 415.
46. H. Niehus, W. Heiland and E. Taglauer, Low-energy ion scattering at surfaces, Surf. Sci. Rep., 1993, 17, 213–303.
47. H. Bay, J. Bohdansky, W. Hofer and J. Roth, Angular distribution and differential sputtering yields for low-energy light-ion irradiation of polycrystalline nickel and tungsten, Appl. Phys., 1980, 21, 327–333.
48. S. A. Khan, K.-H. Heinig and D. Avasthi, Atomistic simulations of Au–silica nanocomposite film growth, J. Appl. Phys., 2011, 109, 094312.
49. C. Pannu, U. B. Singh, S. Kumar, A. Tripathi, D. Kabiraj and D. Avasthi, Engineering the strain in graphene layers with Au decoration, Appl. Surf. Sci., 2014, 308, 193–198.
50. S. Mahieu, W. Leroy, K. van Aeken, M. Wolter, J. Colaux, S. Lucas, G. Abadias, P. Matthys and D. Depla, Sputter deposited transition metal nitrides as back electrode for CIGS solar cells, Sol. Energy, 2011, 85, 538–544.
51. R. C. Weast, Handbook of Chemistry and Physics, 67th edn, Chemical Rubber Co., Cleveland, OH, 1987.

---